A torque sensor is used primarily to detect the transmission of torque through a shafting system. The sensor is utilized in those situations where a torque is being delivered through one loaded set of shafts with a backup set of shafts which are in an unloaded condition. This redundant system has particular applicability in aerospace applications where failure of a primary system, for example, actuation of flight controls, is unacceptable without a backup system to take over torque transmission for actuating on-board systems. Thus, when the primary shafting system fails, the load must be reliably transferred to the secondary shafting system. The torque sensor is critical for advising the system operator that the second shafting system is now carrying a load and that there has been a failure in the primary system.
In the past, several devices have been available to perform a torque sensing function. One such device is a positive engagement torque sensor shown in U.S. Pat. No. 4,597,480. Although such systems have been generally acceptable, they also have characteristic problems which inhibit their ability to function under extreme circumstances such as in high speed shafting systems.
For example, prior art devices have utilized a cantilevered screw and nut arrangement, located in the secondary load path or secondary set of shafts to perform the torque sensing function. Essentially such an arrangement consists of a screw connected to one of the shafts in the secondary load path. This screw is threadably associated with a nut connected to a separate shaft in the secondary load path. When the screw/nut arrangement is allowed to rotate freely no torque can be transmitted across the secondary load path. Typically, the secondary load path shafting will operate at speeds between 1,000 and 18,000 rpm. The secondary shafting system is normally driven by the primary system through transfer gearboxes and creates a complete loop of shafting from the primary drive source through the actuator system and back into the primary drive source, except for the torque sensing device which makes it an open loop during normal operation. In the event that there is an interruption in the primary drive shafting system by way, for example, of a break in the primary drive shaft itself, there will be created a differential motion between the screw and the nut by virtue of one member, either the screw or the nut, continuing to be driven by the intact portion of the primary drive shaft and the other member remaining stationary or being backdriven by the load on the interrupted shafting. As the nut rotates relative to the screw, relative translation between the screw and the nut occurs. As soon as the nut has reached an end stop position on the screw and translation is stopped torque transmission occurs and torque is transmitted through the nut to the screw causing the non-rotating shaft to now be rotated in a controlled manner and being driven off the intact portion of the primary shafting system. A conventional switch arrangement is provided to detect the translation of the nut along the screw caused by the differential rotation of the two secondary load path shafts so as to indicate that the back-up system is transmitting torque load from the primary drive system. The use of the secondary load path prevents any serious structural interruption of the primary shafting system from causing an interruption of the system operation and allows the overall system to continue to function in a normal manner which prevents what could otherwise result in disastrous consequences.
However, in arriving at the present invention, it has been found that when the secondary drive shaft is operating at high speeds, i.e. in the 5,000 rpm range, the unbalanced mass on a cantilevered screw and nut arrangement can result in large oscillatory motion and greater instability during operation which could result in false torque indication and/or premature failure of the sensor.
Moreover, prior torque sensors had lost motion (or dead band) required for sensing motion as the arrangement was moved through the dead band area. Thus, each time torque is reversed on the shaft system the torque sensor elements will have lost motion before transmitting torque to the opposite shaft. Although lost motion during reversing operations is not always a significant disadvantage, it can result in significant position error of elements being driven by the shaft in critical systems.
Threaded mechanisms which produce differential motion between parts are, of course, generally well known. For example, U.S. Pat. No. 4,337,868 shows a telescopic crane boom having rotatable extending, retracting screws which utilize a hollow base section, a hollow intermediate section telescoped within the base section and a hollow fly section telescopable within an intermediate section. Rotatable screws are provided within the boom and are driven simultaneously to effect axial extension of the boom sections. Such an arrangement is not concerned with the problems encountered in high speed torque sensors in which the problems of cantilevering of the screw and nut arrangement can result in catastrophic failure.
Likewise, U.S. Pat. No. 4,381,166, discloses a fork unit having adjustable forks in which rotatably supported threaded rods are provided and carry an elongated nut so that upon rotation of the threaded rods the elongated nuts move axially upon the rods causing desired movement of forks toward or away from each other. Again, such a mechanism is not concerned with the problems encountered in torque sensor applications and does not suggest anything to solve those problems.